The present invention relates to piston pumps and specifically those incorporating a piston which reciprocates in a chamber of cylindrical housing and is disposed coaxially relative to a drive shaft. The piston has a cam surface which is in operative engagement with at least one cam projection having a complementary cam surface connected with the drive shaft and rotating with it.
Piston pumps of this general construction are not new, per se. A typical prior art axial piston pump is shown in German Pat. No. 1,299,189 and in this assembly, the cam surface is arranged on the back side of the piston and is formed of an integral solid material. In these assemblies, the piston is very heavy and therefore has a high mass to accelerate. Additionally, the entire pump is rather elongated in an axial direction. The axial construction length is even further increased in the known constructions by reason of the fact that planetary running rolling elements are disposed between the cam surfaces of the piston and the drive shaft.
Another prior art pump assembly is shown in German published application No. 2,646,340. There is disclosed additionally a vacuum pump with an immediate drive of the pump of a dynamo in which the free end of the shaft of the dynamo has a bent portion and the eccentricity of this bent portion provides the stroke for a piston member of the vacuum pump. The bent portion carries a ball bearing, wherein the outer ring is designed to cooperate with a pressure disc of the piston member. The piston member is in the form of an elastic membrane which limits the stroke of the piston to only one per drive shaft rotation and therefore this type of pump has only a comparatively small forwarding capacity. Furthermore, the bent portion must be produced and machined and is relatively expensive.
With the foregoing in mind, it is an object of the present invention to provide a piston pump of the above mentioned general type which consists of economically and relatively easy to produce components and is compact and space saving at a high capacity. This is achieved in accordance with the present invention by a piston construction consisting of a single or multi component thin-walled cup closed off at one end wherein the cam surface of the piston is arranged inside the center recess of the cup. In its simplest form, the piston assembly of the present invention consists of a single cup closed off on one side which is generally pot shaped. Alternatively, the piston assembly may comprise plural elements consisting of one cup and two thin-walled sleeves arranged axially next to one another in the bore of the cup. In order to form the piston surface, either the cup itself is closed off on one side or in a cylindrical construction of the cup open on both sides, at least one of the sleeves is closed off at one side, that is, one sleeve is provided with a bottom.
The cam surface or cam follower of the piston in accordance with the present invention can be formed by a radially inwardly directed tab which is preferrably formed integrally with the sleeve shaped section of the piston as a bent in radially inwardly directed portion. In a multi-component form of the piston, it is advantageous to provide the pair of thin-walled sleeves disposed axially next to one another in the bore of the cup each with at least one radially inwardly directed tab which forms an axial space therebetween within which engages the edge of the cam disc carrying the cam surfaces rotating with the drive shaft. In a preferred form of the multi-component construction, each of the sleeves preferrably has two diametrically opposed tabs.
A piston constructed in accordance with the present invention can be comprised of components which can be simply and economically produced without machining in a deep drawing process. This piston assembly is lightweight and occupies comparatively little space by reason of the thin-walled thickness of the components. A lightweight piston is especially advantageous to minimize capacity losses of the drive. Furthermore, by reason of this construction, the two sleeves inserted in the cup housing forming the piston may be shaped identically which further reduces tooling costs and substantially simplifies the bearing support of the individual components. Additionally, the tabs forming the cam surface or cam follower of the piston can be simply punched without machining, bent or bent over, so that the machining step or special shaping of the starting material is not necessary.
In accordance with another feature of the present invention, the tabs are shaped so that they are arcuate or curved in a peripheral direction of the cam disc and in this manner form lubricant pockets between the tabs and the corresponding surfaces of the cam disc which reduces friction and wear of the components engaging or sliding on one another. It is also possible to reduce friction in accordance with a preferred embodiment of the invention by coating the tabs and/or the cam surfaces of the cam disc with a layer of low friction material.
In accordance with another feature of the present invention, rather simplified means are provided to secure the piston against rotating movement in relation to the cylindrical housing by providing either the piston at the outer peripheral surface or the bore of the cylindrical housing with at least one radial projection and the bore of the cylinder or the outside surface of the piston with a complementary corresponding recess forming a pocket for a radial projection which engages in the recess in a manner to permit relative axial movement between the piston and the cylindrical housing and restrict rotational movement. Alternatively, the outer peripheral surface of the piston and the corresponding bore of the cylinder may be provided with complementary recesses for rolling elements which serve the same purpose, that is, to permit axial movement of the piston in the housing but restrict rotational movement.
In lieu of the above, since the piston may be formed in a deep drawing process so that it can easily be formed to various non-circular shapes and also since the cylinder bore is machined by reaming, the piston may be secured against rotation by forming it of a non-circular shape, for example, the shape of an ellipse. The cylinder bore also may be formed of a complementary non-circular shape to achieve the desired non-rotation of the piston in the bore of the cylinder.
While the principal embodiment of the invention illustrated is a single acting piston assembly, the principle of the present invention has equal application to double acting assemblies wherein the second stroke volume is defined by the inner front face of the cup like piston. However, since the smallest possible stroke volume cannot be attained in this way, it is proposed according to another feature of the present invention to locate the position of the open rim of the cup radially inwardly up to the area of the drive shaft of the cam disc or to design the sleeve on the side of the drive shaft of the cam disc with a bottom having a suitable opening for the drive shaft. In accordance with this design, the unavoidable gap between the drive shaft, the annular projection of the cam disc and the inside edge of the shifted rim, a suitable seal is provided in the opening in the bottom of the cup. It is also advantageous to seal the stroke volume against the housing wall penetrated by the drive shaft of the cam disc.